1. Field of the Invention
This invention relates to a method and apparatus for testing the AC characteristics of an integrated circuit.
2. Description of the Related Art
In the conventional method for testing the AC characteristics of an integrated circuit (hereinafter abbreviated to IC), in order to examine whether the IC will work correctly or breakdown when it is incorporated in a system, system signals as complex as those used in practical applications, are supplied from system signal generator 602 through measuring board 605 to IC 601 (see FIG. 6). Then, each one of circuits A, B, C, and D in IC 601 is operated in practice,-and the resultant signals output from these circuits are supplied in series and measured in testing apparatus 603.
For example, in order to test an IC of a television set, measuring board 605 has external elements X, as shown in FIG. 7. In addition, a signal generator for generating complicated signals (a video signal or a chroma signal) which are used in the practical operation of the TV set, and an alternating signal tester having an apparatus for measuring the outputs of the measuring board, must be provided for testing the AC characteristics of the IC. Such a tester is very costly.
Since the measuring board must comprise the same external elements as those of the set in which the IC is to be incorporated, enormous effort and time must be spent in manufacturing such a measuring board. Further, in the measuring board having many external elements, a number of electrolytic capacitors of a large capacitance, many plus relays, must be provided. So, the above system has the following drawbacks.
There may be many items included in the measuring conditions for testing AC characteristics. In some cases, these are more than ten items. Whenever the measuring conditions are changed, the voltage across both ends of each of the above-mentioned electrolytic capacitors also changes. Accordingly, charging and discharging of each capacitor takes place, so that there is a waiting time before measuring can be performed.
Because of the many test items, the measuring board has a number of relays as mentioned above: Most of the area of the measuring board is occupied by these relays. As a result, the wiring connecting the elements is long, resulting in degraded electric quality of the measuring board.
Some types of measuring boards include more than 100 elements, such as a crystal, a SAW, a tank coil, and a capacitor. Even if each of the elements is selected carefully, the measuring boards cannot be correlative to one another.
Another problem results from the amplitude of the signal output from the signal source depending on impedance such as the floating capacity of the line which transmits the signal from the IC tester through a test head to the IC to be measured. Accordingly, the designated amplitude may seldom be obtained at the IC to be measured. For this reason, the measuring accuracy is compensated by software if necessary. For example, a route which bypasses the IC is provided just before the IC and an AC measuring apparatus is connected to that route. The value measured by the measuring device is compared to the input value designated in the test program, and any errors are compensated for by the test program.
However, in the above method utilizing software, the frequency characteristic of the line from the IC tester to the IC would be taken into account. In other words, whenever the measuring frequency is changed, the above-described error must be compensated for in order to input a correct signal amplitude over the entirety of a sequence of tests. If the compensation is executed over all tests, the test time will be increased. Thus, in general, the compensation is performed with respect to the representative frequency and therefore it is impossible to obtain accurate measurements over the entire test sequence.
As has been described above, in the conventional testing method, complicated signals must be measured by complicated signal systems. In addition, if the IC has a large number of circuit cells, and there are many cells which must be inspected with respect to one item, a person who is not skilled in the system, the tester, and the IC, cannot analyze the result of the test of each item.